CP violation is the violation of the combined conservation laws associated with charge conjugation (C) and parity (P) by the weak nuclear force, which is responsible for reactions such as the decay of atomic nuclei. Charge conjugation is a mathematical operation that transforms a particle into an antiparticle, for example, changing the sign of the charge. Charge conjugation implies that every charged particle has an oppositely charged antimatter counterpart, or antiparticle. The antiparticle of an electrically neutral particle may be identical to the particle, as in the case of the neutral pi meson, or it may be distinct, as with the antineutron. Parity, or space inversion, is the reflection in the origin of the space coordinates of a particle or particle system; i.e., the three space dimensions x, y, and z become, respectively, -x, -y, and -z. Stated more concretely, parity conservation means that left and right and up and down are indistinguishable in the sense that an atomic nucleus throws off decay products up as often as down and left as often as right.

For years it was assumed that charge conjugation and parity were exact symmetries of elementary processes, namely those involving electromagnetic, strong, and weak interactions. The same was held true for a third operation, time reversal (T), which corresponds to reversal of motion. Invariance under time implies that whenever a motion is allowed by the laws of physics, the reversed motion is also an allowed one. A series of discoveries from the mid-1950s caused physicists to alter significantly their assumptions about the invariance of C, P, and T. An apparent lack of the conservation of parity in the decay of charged K mesons into two or three pi mesons prompted the Chinese-born American theoretical physicists Chen Ning Yang and Tsung-Dao Lee to examine the experimental foundation of parity itself. In 1956 they showed that there was no evidence supporting parity invariance in weak interactions. Experiments conducted the next year verified decisively that parity was violated in the weak interaction beta decay. Moreover, they revealed that charge conjugation symmetry also was broken during this decay process. The discovery that the weak interaction conserves neither charge conjugation nor parity separately, however, led to a quantitative theory establishing combined CP as a symmetry of nature. Physicists reasoned that if CP were invariant, time reversal T would have to remain so as well. But further experiments, carried out in 1964, demonstrated that the electrically neutral K meson, which was thought to break down into three pi mesons, decayed a fraction of the time into only two such particles, thereby violating CP symmetry. CP violation implied nonconservation of T, provided that the long-held CPT theorem was valid. In this theorem, regarded as one of the basic principles of quantum field theory, charge conjugation, parity, and time reversal are applied together. As a combination, these symmetries constitute an exact symmetry of all types of fundamental interactions.

No completely satisfactory explanation of CP violation has yet been devised. The size of the effect, only about two parts per thousand, has prompted a theory that invokes a new force, called the "superweak" force, to explain the phenomenon. This force, much weaker than the nuclear weak force, is thought to be observable only in the K-meson system or in the neutron's electric dipole moment, which measures the average size and direction of the separation between charged constituents. Another theory, named the Kobayashi-Maskawa model after its inventors, posits certain quantum mechanical effects in the weak force between quarks as the cause of CP violation.

The attractive aspect of the superweak model is that it uses only one variable, the size of the force, to explain everything. Furthermore, the model is consistent with all measurements of CP violation and its properties. The Kobayashi-Maskawa model is more complicated, but it does explain CP violation in terms of known forces.

CP violation has important theoretical consequences. The violation of CP symmetry, taken as a kind of proof of the CPT theorem, enables physicists to make an absolute distinction between matter and antimatter. The distinction between matter and antimatter may have profound implications for cosmology. One of the unsolved theoretical questions in physics is why the universe is made chiefly of matter. With a series of debatable but plausible assumptions, it can be demonstrated that the observed matter-antimatter ratio may have been produced by the occurrence of CP violation in the first seconds after the " big bang," the violent explosion that is thought to have resulted in the formation of the universe (see big-bang model).

The American Institute of Physics Bulletin of Physics News Number 420 March 29, 1999 by Phillip F. Schewe and Ben Stein

Direct CP violation has been observed at Fermilab by the KTeV collaboration. An important way of apprehending the basic nature of time and space (in the finest tradition of Greek philosophy) is to ask "what if" questions. For example, will a collision between particles be altered if we view the whole thing in a mirror? Or what if we turn all the particles into antiparticles? These propositions, called respectively parity (P) and charge conjugation (C) conservation, are upheld by all the forces of nature except the weak nuclear force. And even the weak force usually conserves the compound proposition of CP. In only one small corner of physics---the decay of K mesons---has CP violation been observed, although physicists suspect that CP violation must somehow operate on a large scale since it undoubtedly helped bring about the present-day preponderance of matter over antimatter.

K mesons (kaons) are unstable and do not exist outside the interiors of neutron stars and particle accelerators, where they are artificially spawned in K-antiK pairs amidst high energy collisions. K's might be born courtesy of the strong nuclear force, but the rest of their short lives are under control of the weak force, which compels a sort of split personality: neither the K nor anti-K leads a life of its own. Instead each transforms repeatedly into the other. A more practical way of viewing the matter is to suppose that the K and anti-K are each a combination of two other particles, a short lived entity called K1 which usually decays to two pions (giving K1 a CP value of +1) and a longer-lived entity, K2, which decays into three pions (giving K2 a CP value of -1). This bit of bookkeeping enshrined the idea then current that CP is conserved.

All of this was overthrown when in 1964 the experiment of Jim Cronin and Val Fitch showed that a small fraction of the time (about one case in every 500, a fraction called epsilon) the K2 turns into a K1, which subsequently decays into two pions. This form of CP violation is said to be indirect since the violationoccurs in the way that K's mix with each other and not in the way that K's decay. One theoretical response was to say that this lone CP indiscretion was not the work of the weak force but of some other novel "superweak" force. Most theorists came to believe, however, that the weak force was responsible and, moreover, that CP violation should manifest itself directly in the decay of K2 into two pions. The strength of this direct CP violation, characterized by the parameter epsilon prime, would be far weaker than the indirect version. For twenty years detecting a nonzero value of epsilon prime has been the object of large-scale experiments at Fermilab and for nearly as long at CERN. In each case, beams of K's are sent down long pipes in which the K-decay pions could be culled in sensitive detectors.

At the APS Centennial meeting in Atlanta last week, both groups discussed their work. The KTeV group at Fermilab reported a definite result: a ratio of epsilon prime to epsilon equal to 28 (+/- 4) x 10^-4, larger than the theoretical expectation. As for the NA48 group at CERN, Lydia Iconomidou-Fayard (lyfayard@in2p3.fr) said that data analysis was still proceeding and no definite measurement could be reported at this time. The principal conclusion was stated by KTeV co-spokesman Bruce Winstein (bruce@uchep.uchicago.edu, 773-702-7594): Before the new experiments direct CP violation had not been established, owing to the large uncertainty in the early measurements of epsilon prime; the new experiment, by contrast, does succeed in establishing a nonzero value for epsilon prime, thus providing a new way to probe (a parameter that can be measured in the lab) this cosmologically-important and most mysterious feature of particle physics.

(a) The neutral K meson and its antimatter counterpart can both be thought of as a combination of a short-lived particle K1 (green squiggle) which mostly decays into two pions (each indicated by the letter p) and a long-lived particle K2 (red squiggle) which decays mostly into three pions. (b) In some rare cases, however, the K2 (CP= -1) turns into a K1 (CP= 1), which then decays into two pions. This is evidence for indirect CP violation. (c) To illustrate how K mixing comes about, consider the analogy with polarized light. Ordinary light from the sun contains light of all different polarizations (the direction of the light wave's electric field). But if the light is passed through a Polaroid filter oriented vertically, some of the light will be blocked and only that portion with a vertical polarization will emerge. In this beam there can be no light with a horizontal orientation. Next pass the light through a filter oriented at 45 degrees to the vertical. The light that emerges (at even lesser intensity) will now be oriented at the same 45 degrees; this light can be said to have a component which has vertical polarization and a component with horizontal polarization. The proof that some of the beam is now horizontally polarized (whereas a moment before the light was exclusively vertical) is that some light does emerge from a third polarizer oriented horizontally. Something like this is at work in converting K1's and K2's into each other just as vertically polarized light is turned into horizontal. Instead of polarizers, however, the K's are made to pass through thin slabs of matter, in which beams of short-lived K's are "regenerated" from beams of pure long-lived K's. (d) The recently observed case in which K2's are seen to be decaying directly into two pions. This is evidence of direct CP violation.

Excerpt from the Encyclopedia Britannica without permission.